1. Field of the Invention
The present invention is directed to a method for operating a magnetic resonance tomography apparatus having a gradient magnet system, of the type wherein a sequence of digital signals are transmitted to a digital-to-analog converter and wherein the digital-to-analog converter, for each digital signal supplied thereto, generates a gradient current pulse which is supplied to a gradient coil to produce a gradient magnetic field in the magnetic resonance tomography apparatus,
2. Description of the Prior Art
In the context of the operation of digital-to-analog converters, it is generally known that a quantization error or quantization uncertainty is inherent in the conversion process. This is because the emitted analog signal, which is continuous, nevertheless can assume only discrete values as a result of the conversion process. Successive values of the analog signal can differ, at a minimum, by the step size of the amplitude increase (or decrease) associated with a bit of the incoming digital signal. Therefore, as long as the digital signal supplied to the digital-to-analog converter is within a bandwidth (range) of one-half of the minimum step size to the nearest analog signal which can be emitted, that analog signal will be emitted.
In the context of digital-to-analog converters which are used to produce gradient pulses in magnetic resonance tomography apparatus, the deviations between the xe2x80x9ctheoreticalxe2x80x9d analog signal to be emitted, and the analog signal which is actually emitted by the converter, particularly with respect to low signal levels, can produce errors which are not negligible in the subsequent image reconstruction.
It is an object of the present invention to provide a method for operating a magnetic resonance tomography apparatus wherein the aforementioned quantization error is reduced or substantially eliminated.
The above object is achieved in accordance with the principles of the present invention in a method for operating a magnetic resonance tomography apparatus having a digital-to-analog converter of the above type, wherein the analog signal emitted by the converter is converted into a digital signal and this digital signal is compared to the incoming digital signal which was used to produce the analog signal fin the converter. Any difference between these two digital signals which is identified as a result of the comparison is added to the next incoming digital signal which is to be converted.
As a result of the inventive method, the aforementioned quantization errors cannot accumulate overtime. The phase error of the magnetic resonance signal, which leads to errors in the image reconstruction, is dependent on the total error integrated (accumulated) over time. Since such accumulation cannot occur, such phase errors do not reach a level which can produce errors in the image reconstruction. The error feedback in accordance with the inventive method, therefore, minimizes or completely eliminates the quantization error, without the need for more complicated techniques, such as over-sampling or higher resolution digital-to-analog conversion.
The inventive method is particularly effective when some of the digital signals are in the order of magnitude of the minimum step size.
The method is preferably applied to rapidly operating digital-to-analog converters, such as when the sequence of digital signals is transmitted to the digital-to-analog converter, and the analog signal is emitted by the digital-to-analog converter, with a time clock of one millisecond at a maximum, preferably below 0.1 millisecond, for example, 0.01 millisecond.
Applying the inventive method only to small digital signals is particularly expedient when the sequence of digital signals is composed of a first sequence of digital signals and second sequence of digital signals, with the second sequence of digital signals being emitted following the first sequence of digital signals, and wherein the digital signals in the second sequence are significantly lower than the digital signals in the first sequence.
In the production of gradient pulses, it is normally the case that the digital signals of the first sequence are composed of nominal signals and compensation signals, and the digital signals in the second sequence are composed only of compensation signals. The nominal signals are used by the coil to generate a gradient magnetic field, and the compensation signals compensate for noise fields which the gradient magnetic field produces in components surrounding the gradient coils.